<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <script>
      var num = 1;
      function demo() {
        console.log(num);
        function demoSon() {
            console.log(num);
          num = 3;
            console.log(num);
        }
        var num = 2;
        demoSon();
      }
      demo();
      //   等价于如下代码
      //     var num;
      //     function demo() {
      //       function demoSon() {
      //         console.log(num); //2
      //         num = 3;
      //         console.log(num); //3
      //       }
      //       var num;
      //       console.log(num); //undefind
      //       num = 2;
      //       demoSon();
      //     }
      //     num = 1;
      //     demo();
    </script>
  </body>
</html>
